Efficiency Enhancement for Natural Gas Liquefaction with CO2 Capture and Sequestration Through Cycles Innovation and Process Optimization
by Abdullah Alabdulkarem
Liquefied natural gas (LNG) plants are energy intensive. As a result, the power plants operating these LNG plants emit high amounts of CO2. To mitigate global warming that is caused by the increase in atmospheric CO2, CO2 capture and sequestration (CCS) using amine absorption is proposed. However, the major challenge of implementing this CCS system is the associated power requirement, increasing power consumption by about 15-25%. Therefore, the main scope of this work is to tackle this challenge by minimizing CCS power consumption as well as that of the entire LNG plant though system integration and rigorous optimization.
The power consumption of the LNG plant was reduced through improving the process of liquefaction itself. In this work, a genetic algorithm (GA) was used to optimize a propane pre-cooled mixed-refrigerant (C3-MR) LNG plant modeled using HYSYS software. An optimization platform coupling Matlab with HYSYS was developed. New refrigerant mixtures were found, with savings in power consumption as high as 13%. LNG plants optimization with variable natural gas feed compositions was addressed and the solution was proposed through applying robust optimization techniques, resulting in a robust refrigerant which can liquefy a range of natural gas feeds.
The second approach for reducing the power consumption is through process integration and waste heat utilization in the integrated CCS system. Four waste heat sources and six potential uses were uncovered and evaluated using HYSYS software. The developed models were verified against experimental data from the literature with good agreement. Net available power enhancement in one of the proposed CCS configuration is 16% more than the conventional CCS configuration.
To reduce the CO2 pressurization power into a well for enhanced oil recovery (EOR) applications, five CO2 pressurization methods were explored. New CO2 liquefaction cycles were developed and modeled using HYSYS software. One of the developed liquefaction cycles using NH3 as a refrigerant resulted in 5% less power consumption than the conventional multi-stage compression cycle.
Finally, a new concept of providing the CO2 regeneration heat is proposed. The proposed concept is using a heat pump to provide the regeneration heat as well as process heat and CO2 liquefaction heat. Seven configurations of heat pumps integrated with CCS were developed. One of the heat pumps consumes 24% less power than the conventional system or 59% less total equivalent power demand than the conventional system with steam extraction and CO2 compression.
Doctoral Dissertation
http://hdl.handle.net/1903/15203
Characterization of Heat Transfer and Pressure Drop of Normal Flow Heat Exchangers in Counter Flow Configuration
by Rohit Subhash Andhare
In today's times, successful technology advancement lies in making systems that are highly compact, offer superior energy efficiency, while sustainable and cost effective . There is interest in developing small heat exchangers having better flow distribution control rather than bulky heat exchangers which are energy intensive. Microchannels and microreactors controlled by microprocessors are slowly taking over energy conversion, transportation and process industry. The nature inspired - Fractal arrangement of manifold-microchannels has the potential to provide enormous heat transfer capabilities at an attractive coefficient of performance. However majority of such fractal flow manifolds are very short and operate with short counterpart microchannel. They have not been completely adopted for counter flow configuration required by majority of the industrial processes. The work covered under this thesis is focused on adopting of high performance fractal microchannel arrangement to counter flow configuration heat exchangers that are required by industrial processes. Two single phase solution heat exchangers were developed using this approach. The solution heat exchanger is an essential component in absorption refrigeration cycle to convert waste heat into cooling. The study also utilized the novel additive manufacturing process of 3D printing to develop a tubular manifold in order to promote the fractal normal flow on tubular surfaces. The heat exchangers developed as a part of this thesis show enhancement in the overall performance and demonstrate high potential of the proposed technology.
Master's Thesis
http://hdl.handle.net/1903/15224
Design and Experimentation of a Manufacturable Solid Desiccant Wheel Assisted Separate Sensible and Latent Cooling Packaged Terminal Air Conditioning System
by Michael Vincent Cristiano
Packaged terminal air conditioning (PTAC) systems are typically utilized for space heating and cooling in hotels and apartment buildings. However, in an effort to reach comfortable relative humidity in the conditioned space, they cool the air to its dew point and some reheating may be required to reach room set point temperature. In this study a commercial prototype of a solid desiccant wheel assisted separate sensible and latent cooling (SSLC) packaged terminal air conditioning (PTAC) system was designed . This iteration of the SSLC prototype was modeled based on PTAC type air conditioning units and was designed to achieve a 30% increase in system efficiency over current commercially available PTAC units. Heat exchangers used as evaporator and condenser were modeled in Coildesigner and VapCyc was used for system level modeling. Also a test facility was constructed in order to evaluate the performance of the proposed SSLC unit. Shakedown testing was conducted under various operating conditions in order to compare the SSLC system to a standard PTAC unit without desiccant wheel. With the necessary adjustments to the experimental prototype, the system could increase the overall capacity through latent cooling with negligible additional power consumption.
http://hdl.handle.net/1903/15168
Enhanced Gas-Liquid Absorption Utilizing Micro-Structured Surfaces and Fluid Delivery Systems
by Harish Ganapathy
Despite intensive research and development efforts in renewable energy in recent years, more than 80% of the energy supply in the year 2040 is expected to come from fossil fuel-based sources. Increasing anthropogenic greenhouse gas emissions led the United States to legislatively limit domestic CO2 emissions to between 1000-1100 lb/MWh for new fossil fuel-fired power plants, thus creating an urgent need for efficient gas separation (capture) processes. Meanwhile, the gradual replacement of coal with cleaner burning natural gas will introduce additional challenges of its own since nearly 40% of the world's gas reserves are sour due to high concentrations of corrosive and toxic H2S and CO2 gases, both of which are to be separated. Next-generation micro-structured reactors for industrial mass and heat transfer processes are a disruptive technology that could yield substantial process intensification, size reduction, increased process control and safety. This dissertation proposes a transformative gas separation solution utilizing advanced micro-structured surfaces and gas delivery manifolds that serves to enhance gas separation processes. Experimental and numerical approaches have been used to achieve aggressive enhancements for a solvent-based CO2 absorption process. A laboratory-scale microreactor was investigated to fundamentally understand the physics of multiphase fluid flow with chemical reactions at the length scales under consideration. Reactor design parameters that promote rapid gas separation were studied. Computational fluid dynamics was used to develop inexpensive stationary (fixed) interface models for incorporation with optimization engines, as well as high fidelity unsteady (deforming) interface models featuring universal flow regime predictive capabilities. Scalability was investigated by developing a multiport microreactor and a stacked multiport microreactor that represented one and two orders magnitude increase in throughput, respectively. The present reactors achieved mass transfer coefficients as high as 400 1/s, which is between 2-4 orders of magnitude higher than conventional gas separation technologies and can be attributed to the impressive interfacial contact areas as high as 15,000 m2/m3 realized in this study through innovative design of the system. The substantial enhancement in performance achieved is indicative of the high level of process intensification that can be attained using the proposed micro-structured reactors for gas separation processes for diverse energy engineering applications. This dissertation is the first comprehensive work on the application of micro-structured surfaces and fluid delivery systems for gas separation and gas sweetening applications. More than ten refereed technical publications have resulted from this work, part of which has already been widely received by the community.
http://hdl.handle.net/1903/15861
Development of Advanced Numerical Models for Variable Geometry Microchannel Heat Exchangers
by Long Huang
Air-to-refrigerant microchannel heat exchangers (MCHXs) are now extensively used in the heating, ventilation, air-conditioning and refrigeration (HVAC&R) industry. Numerical models are favored in the research and development process due to the fast calculation speed and lower cost as opposed to prototype development and testing. More recently, the evolving simulation and manufacturing capabilities have given the engineers new opportunities in pursuing complex and cost-efficient novel heat exchanger designs. Advanced heat exchanger modeling tools are desired to explore geometries out of conventional boundaries of design.
The current research and development of MCHXs has reached a plateau, in that, the optimum designs cannot be further improved with the limited number of geometry related design variables currently used. Freeing up the current MCHX uniform geometry restriction would lead to novel designs that address various design and applications objectives, such as performance enhancement, material reduction and space constraints.
This thesis presents the research, development and comprehensive validation of advanced heat exchanger models for microchannel heat exchangers. These new models include unprecedented modeling capabilities, with extensive consideration of various underlying heat transfer and fluid flow phenomena. The proposed microchannel heat exchanger models are capable of simulating variable geometry microchannel heat exchangers with variable tubes, ports and fins while accounting for effects such as heat conduction, combined heat and mass transfer as well as air and refrigerant flow mal-distribution, thus distinguishing itself as the cutting edge modeling tool in the open literature.
The models are validated against 247 MCHX experimental data points obtained from open literature, in-house laboratories and industry partners. This is the most comprehensive validation of microchannel heat exchanger models in open literature, including eight different fluids and eighteen different geometries. The validated model is then coupled with a multi-objective genetic algorithm to optimize the variable geometry heat exchangers to minimize material and envelope volume. The optimization study shows up to 35 percent reduction in material and 43 percent savings in envelope volume for the same performance compared to a baseline conventional geometry design. This research will be help engineers to develop creative microchannel heat exchangers ultimately resulting in improved systems efficiency at lower costs.
http://hdl.handle.net/1903/16100
Simulation and Analysis of Energy Consumption for an Energy-Intensive Academic Research Building
by Jared Michael Levy
The University of Maryland's Jeong H. Kim Engineering Building is a state-of-the-art academic research facility. This thesis describes an energy analysis and simulation study that serves to identify energy saving opportunities and optimum operation of the building to achieve its goals of high energy efficiency and substantial CO2 emission reduction. A utility analysis, including a benchmarking study, was completed to gauge the performance of the facility and a detailed energy model was developed using EnergyPlus to mimic current operation. The baseline energy model was then used to simulate eight energy efficiency measures for a combined energy savings of 16,760 MMBtu, reducing annual energy use by 25.3%. The simple payback period for the proposed measures as a single project is estimated to be less than one year. Due to the high-tech and unique usage of the Kim Engineering Building, including cleanrooms and research labs, this thesis also contributes to the development of energy consumption benchmarking data available for such facilities.
http://hdl.handle.net/1903/15955
Performance enhancement of heat exchanger coolers with evaporative cooling
by Sahil Popli
Air or water cooled heat exchangers (HX) are typically utilized as condensers or coolers for air-conditioning, refrigeration or process cooling applications in both commercial and industrial sector. However, air cooled heat exchanger performance degrades considerably with rise in ambient air temperature and water cooled coolers require considerable pumping power, a cooling tower and may consume a significant amount of water which may come from fresh water sources. Evaporative cooling offers a unique solution to this problem, where a small amount of wetting water evaporates on HX surface to boost performance in high ambient air temperature conditions.
In this study, several evaporative cooling technologies were applied to three wavy-fin HXs to quantify capacity enhancement ratio (CER) and air-side pressure drop penalty ratio (PRΔPa) compared to respective dry case baseline values. Effect of varying wetting water flow rate, air velocity, fin spacing, hydrophilic coatings, spray orientation and inlet air temperature and relative humidity was investigated on hybrid heat exchanger performance. Several new performance comparison parameters were defined to compare different evaporative cooling approaches.
Deluge cooling achieved overall highest CER but at a PRΔPa that was similar in magnitude to the CER. This limitation was found to be inherent to the nature of wetting water distribution method itself. Although front spray cooling tests indicated PRΔPa~1, front spray evaporative cooling technology was found to have up to 23-75 % lower CER at 60-100% lower PRΔPa compared to deluge cooling. In order to understand the wetting behavior a novel visualization method was proposed and implemented, which consisted of borescope assisted flow mapping of water distribution within the HX core as a function of air velocities and wetting water flow rates. It was found that up to 85% of HX volume remained dry during front spray cooling which accounted for lower capacity enhancement and deluge cooling forms non-uniform and thick water film which causes bridging and increased PRΔPa, A larger component level testing with HX size similar to commercial units allowed to identify constraints of different evaporative cooling methods, which would not be possible if tests were performed at a smaller segment or fin level.
A novel spray cooling technology utilizing internal jet spray cooling within HX volume was both proposed and implemented and a provisional patent # 61/782,825 was obtained. Compared to front spray cooling at a given spray rate, internal spray cooling could either achieve up to 35% higher HX cooler capacity, or obtain same HX cooler capacity at approximately three times lower air-side pressure drop. Alternatively, at same air-side pressure drop wetting water savings of up to 68-97% are achieved. Internal spraying combines advantages of conventional technologies and overcomes the drawbacks, by getting CER of approx. 3.8, without film carryover and at PRΔPa=1, while getting maximum wetting uniformity. Intermittent cooling combined with internal spraying could reduce water consumption as evaporative cooling sustains though the brief period when spray is turned off. Thus, potential for significant energy and water savings, targeted cooling, and retrofit design offers significant commercialization opportunity for future hybrid evaporative coolers. Discussions are underway for the inclusion of this technology into product line up of a leading HX manufacturing company.
Transient Modeling of Two-Stage and Variable Refrigerant Flow Vapor Compression Systems With Frosting and Defrosting
by Hongtao Qiao
This thesis presents the development of an advanced modeling framework for the transient simulation of vapor compression systems. This framework contains a wide range of components and its modular nature enables an arbitrary cycle configuration to be analyzed. One of the highlights of this framework is the first-principles heat exchanger models with many salient simulation capabilities. Specifically, a high-order discretized model employing finite volume analysis is developed based on a decoupled approach to modeling the heat transfer and pressure drop performance of the heat exchanger. The frosting and defrosting models developed in the thesis are integrated into this heat exchanger model, allowing more accurate performance assessment of heat pumps. Meanwhile, an advanced low-order moving boundary heat exchanger model is developed with switched model representations to accommodate the changing numbers of fluid zones under large disturbances. Compared to the existing moving boundary models in the literature, this new model accounts for refrigerant pressure drop and possesses a more accurate evaluation for the air side heat transfer.
Based on this modeling framework, the transient characteristics of a flash tank vapor injection (FTVI) heat pump system undergoing cycling, frosting and reverse-cycle defrosting operations are thoroughly explored. The dynamic system response when subjected to a step change in the opening of the upper-stage electronic expansion valve is also investigated. Comparison between the predictions and experimental data shows that the simulation can adequately capture the transient heat transfer and fluid flow phenomena of the system and thus demonstrating the fidelity of the models. Furthermore, a pull-down simulation for a multi-split variable refrigerant flow (VRF) air-conditioning system with six indoor units has been carried out. Control strategy that aims to maintain the indoor air temperatures at set values is proposed. The simulation test for controllability shows that the proposed control strategy is feasible to achieve the temperature control of individual zones.
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